This invention relates generally to the testing art, and more particularly to a unique simultaneous incremental strain/incremental temperature analog drive for, and a method of, testing and evaluating the stress response of a solid propellant under thermally induced loading conditions (i.e., axial compressive or tensile loading) which directly simulate those conditions occurring in a case-bonded solid rocket grain. For those not of the art, it is here to be noted that a solid rocket grain is a configured solid propellant.
The stress-free temperature (T.sub.SF) of a solid rocket grain is defined as that temperature where thermally induced stresses are zero. For the freshly cured propellant that temperature falls a few degrees above the cure temperature, with the shift being due to a small cure shrinkage. It is standard practice in the art to use the latter temperature as the reference point for all solid rocket grain stress analyses, regardless of the environmental exposures involved. Unfortunately, however, actual laboratory measurements have established that shifts in T.sub.SF ranging from 60 percent to 100 percent of the differential between the cure temperature and the storage temperature may not be unusual. The problem assooiated with shifts of this magnitude lies in the fact that any stress analysis of the solid rocket grain, regardless of the level of sophistication of the analysis, will be invalidated if it assumes the solid rocket grain to be stress free at the cure temperature. This is particularly serious if the rocket motor in which the solid propellant grain is incorporated is stored at some elevated temperature, because in such a situation the actual stresses developed during subsequent cooling will be higher than predicted. The shifts to lower temperature, which can occur under temperate or controlled storage conditions, may, upon first consideration, appear to be beneficial, but they could cause unacceptable bond shear stresses to develop upon subsequent heating of the solid rocket grain. Shifts in either direction could cause serious errors in rocket motor life predictions, if not properly taken in account.
In addition to the above, significant shifts in stress-free temperature in a stored rocket motor can completely negate the beneficial effects of motor ambient cure and of high pressure cure. Both of these processing techniques have been used extensively in attempts to reduce the structural requirements imposed on case-bonded solid rocket grains.
If one recognizes the significance of the stress-free temperature shift phenomenon to structural and aging evaluations of a solid propellant, then it becomes readily apparent that what is needed in the art and is not presently available is a laboratory means which will permit reliable measurement of T.sub.SF shift behavior. Stated another way, what is needed is an arrangement (i.e., device and/or method) which permits the laboratory evaluation of a preselected solid propellant's stress response under various theormomechanically coupled loading conditions which closely simulate those to which the case-bonded solid rocket grain of that solid propellant is exposed during tactical motor deployment.